Basics of sliding metallic-bearing materials - Part 5

This is the fifth of a series on principles and characteristics of metallic plain bearing materials and how to optimize them in given uses. Part 1 appeared in June 1994, 2 in July 1994, 3 in Aug. 1994, and 4 in Nov. 1994

GEORGE R. KINGSBURY | Aug 01, 2000

In Part 4, we discussed multilayer bearing-material systems. Now, we cover casting, powder-metallurgy, and roll bonding processes for manufacturing bearing-material systems in single and multilayer types.

Casting processes

Single-metal systems. Except for porous- metal, oil-impregnated bushings, all single-metal systems in Table 1 are commercially produced by casting, with or without subsequent mechanical working. Plate, strip, and sheet forms of commercial bronze, of low-lead and lead-free tin bronzes, and of aluminum-tin alloys are initially cast as ingots, slabs, or bars. Methods are static and continuous casting similar to those for other brass and aluminum mill products. Subsequent rolling and annealing operations are also similar to those of conventional mill products. Because of extreme hot shortness of leaded tin bronzes and aluminumtin alloys, they must be rolled either cold or at only slightly elevated temperature, with frequent intermediate annealing.

The recrystallized wrought structures of bronze and aluminum-tin bearing alloys differ substantially from the initial cast structures, with respect to the configurations of copper and aluminum phases and of free-lead and free-tin phases. Improvements in ductility and forming characteristics that come from these structural changes are important in subsequent bearing manufacture. The changes have no strong effect on performance properties. As-cast and wrought forms of these alloys are in commercial use and equally acceptable in bearings. Tubular and cylindrical bronze, zinc, and aluminum-tin alloy shapes are produced by static, centrifugal, and continuous casting methods, and subsequently machined into bearings. High-lead bronzes are used only as-cast, because of their low ductility and extreme hot shortness, which preclude any substantial plastic deformation of cast shapes. Cast aluminum-tin alloy tubes can withstand limited cold work, however, and in some instances cold compression of 4 to 5% increases yield strength and improves press-fit retention in finished bearings.

Bimetal systems. Specialized casting methods are widely used to produce bimetal materials in tubes and flat strips. Except for aluminum alloy systems, Table 2, classes 3 and 8, all commercial bimetal systems, at least in principle, can be produced by casting. Systems with tin and lead babbitt liners more than about 0.004 in. thick are universally cast.

• Babbitt centrifugal casting. Short tubular steel and bronze shapes (bearing shells) are commonly lined with tin or lead alloys by various forms of centrifugal casting. Here, a machined steel or bronze shell is preheated and coated by immersion in molten tin or tin alloy. The prepared shell is then placed in a lathe-like “spinner” and turned about its axis at controlled speed. Molten babbitt is admitted through one end and uniformly distributed around the inside wall of the shell by centrifugal action. The layer then is cooled and solidified by spraying water against the outside of the rotating shell. These processes can make fine-grain, uniformly thick liner layers, fully bonded to the steel or bronze bearing-back material. Centrifugal casting is especially suited to large-diameter, thick-wall bearings which are made in small quantities, and to full-round seamless bearings that cannot be made from flat strip.

• Bronze centrifugal casting. Leaded tin bronzes also can be applied to the inner walls of steel shells by centrifugal casting. There are various methods of shell preparation, including both molten salt and controlled-atmosphere preheating. Total absence of oxidation of the steel shell’s inner wall is a fundamental requirement for complete metallurgical bonding. Centrifugal casting of bronzes succeeds best with alloys containing more than about 3% tin and not more than about 20% lead. Outside this range, leaded tin bronze and copper-lead alloys are sensitive to lead segregation and consequent nonuniform “centrifuged” microstructures. But inside this range and with well-controlled process conditions, mechanically sound, well-bonded bronze layers with uniform microstructures can be produced.

• Bronze gravity casting. All copperlead alloys and leaded bronzes containing up to about 35% lead can be cast in and bonded to steel shells by gravity casting, in which there is no centrifugal force. In such processes, a core usually forms an annular space inside the shell, into which molten bronze or copper-lead alloy is poured. Many such processes are in commercial use, using a variety of preheating methods, core materials, pouring methods, and quenching procedures.

As in centrifugal casting, absence of oxides on the inner shell wall is necessary for complete bond of alloy layer and steel back. Liner microstructures made by gravity shell casting generally are more uniform than those by centrifugal casting. For low-tin or high-lead compositions, or both, gravity casting is preferred; there is no centrifuging effect on the solidifying alloy.

• Babbitt strip casting. Steel-backed tin alloy and lead alloy bearing-strip materials are commonly produced by continuous casting. It is done on process lines in which separate cleaning, etching, hot tinning, liner-alloy casting, and quenching are carried out continuously on a moving steel bearing-back strip. In-line machining may be included so the strip emerges with closely controlled thickness, suitable for bearing fabrication.

• Bronze strip and slab casting. The oldest commercial processes for steelback copper-lead and leaded-bronze bearing strip also use continuous casting on a moving steel strip. Steel preheating, alloy casting, and quenching are done in a strongly reducing atmosphere to ensure freedom from oxidation. Some in-line machining also can be done, but the cast strip usually is machined in a separate line for close thickness control. There can be additional cold rolling and annealing, especially with the low and medium lead-tin bronze alloys, in which recrystallized structures are frequently preferred for their superior fabrication properties.

Strip casting of copper alloys is difficult, requiring close process control, high operator skill, and expensive special equipment. Only a few bearing manufacturers use it, but with much success. It is used not only for thin gage coiled materials, but also for heavy-gage slabs with steel thicknesses to 0.60 in.

Trimetal systems. Trimetal materials with thick surface layers, Table 3, classes 1, 2, and 3, are used mostly in large bearings. These are produced in low volumes from steel shells initially lined by casting with copper-lead alloys or bronze. After intermediate machining to remove excess liner alloy, such shells are commonly relined with tin or lead babbitt by centrifugal casting. Methods used are essentially the same as those for casting in bare steel or solid bronze shells.

Powder metallurgy processes

Single-metal systems. The only commercial use of powder metallurgy (P/M) methods for making single-metal bearing materials is in fabrication of copper-base and iron-base porous metal bushings, which are subsequently impregnated with oil. The methods are similar to those for making structural P/M shapes — that is, pressing in a closed die and sintering in a reducing atmosphere. Bars, tubes, and finished parts are made in this way. Often, post-sinter coining and re-pressing control final dimensions.

See the “Powder Metallurgy” section of the ASM Metals Handbook, ASM International, Materials Park, Ohio, for detailed information on this technology.

Bimetal and trimetal systems. No powder metallurgy process is in commercial use with lead-base or tin-base bearing alloys, nor is there any commercial process for lining bearing shells by powder metallurgy methods. In manufacture of steel-back copper-lead alloy and leaded bronze strip, however, powder metallurgy methods are used more extensively than any other.

Continuous sintering on a steel backing strip can produce a variety of steelback copper alloy materials, including counterparts of all cast copper-lead and leaded bronze bearing alloys, Table 2, class 4. Here, prealloyed powder particles are spread uniformly onto a moving steel strip. As the strip passes through a furnace in a reducing atmosphere, the particles sinter together, forming an open grid bonded to the strip. After cooling, this bimetal is rolled to densify the liner alloy, then resintered to develop complete interparticle and alloy-to-steel bonds. After resintering, the strip may receive further rolling, to attain finish stock size and, sometimes, to strain-harden the alloy liner for higher strength.

Strip sintering permits production of steel-core “sandwich” material, which is especially suitable for applications requiring two bearing surfaces, such as in some thrust washers. Here, powder spreading, sintering, cooling, and rolling are repeated on the opposite side, after which the strip is finally resintered. Sintered strip for most automotive and truck bearings is processed in coils up to about 0.2 in. thick. Thick-wall materials with steel layers up to about 5/8 in. thick also can be processed in flat slab lengths.

Both bimetal and trimetal bearing materials also can be made by impregnation or infiltration of a lower-melting lead alloy into a layer of sintered copper alloy powder. In impregnation, a bilayer strip of prealloyed copper-lead alloy or leaded bronze powder is immersed in a molten lead-tin alloy bath heated above the melting point of lead. During immersion, the lead-tin alloy replaces some lead at the strip surface. In infiltration, the copper alloy powder layer is free-sintered and not compacted after sintering. The opengrid sintered layer is then infiltrated with material having a lower melting temperature than that of the grid alloy.

The infiltrant is usually molten lead or a lead alloy but it may be a nonmetallic material such as PTFE, which can be introduced in paste or slurry form. A useful class of self-lubricating trilayer structures is made commercially this way — where a PTFE-based infiltrant also forms a thin low-shear-strength surface film.

Powder rolling. A useful application of direct powder rolling developed commercially for plain bearings is production of an aluminum-lead alloy strip for subsequent bonding to a steel back. (See the third item under class 3, Table 2.) Here, prealloyed lead-aluminum powder and unalloyed aluminum powder are fed separately to a powder rolling mill and continuously compacted into a bilayer aluminum strip. After sintering, this strip is roll-bonded to low-carbon steel, with the unalloyed aluminum bonding layer next to the steel. This steel-back strip serves as a bimetal material for bearings, where unit load exceeds the capacity of tin or lead babbitt bimetal.

In all roll bonding — batch or continuous — a rolling mill forces clean aluminum and steel surfaces together at intense pressure, so solid-phase bonding (cold welding) can occur between the two metals at many interface sites. Heat may be applied simultaneously with pressure in hot rolling or subsequently in postroll annealing. It helps develop complete diffusion bonding from initial weld sites and recrystallize the aluminum alloys. Thus, the final bimetallic strip has useful lineralloy ductility and full bonding. Because of undesirable interactions between the free tin constituent and the steel backing, tin-aluminum alloys usually are not bonded directly to steel. A layer of electrolytic nickel plating on the steel surface is commonly used to alleviate these effects with both low-tin and high-tin alloy compositions.

Another common method with tin-aluminum alloys uses a tin-free aluminum interlayer. This is done by using alclad tin-aluminum alloy strip. The tin-free cladding layer serves as the bonding surface and is present as a distinct bond interlayer in the finished bimetal strip.

Direct roll bonding to steel is used most commonly with tin-free aluminum alloys (fifth item under class 3, Table 2), and with lead-aluminum strip materials.

George R. Kingsbury, P.E., recently retired as Senior Engineer from Glacier Vandervell Inc., a major producer of metal plain bearings, is principal of his own metallurgical engineering consulting practice in Lyndhurst (Cleveland), Ohio. He is well known in the bearing materials field as an author, lecturer, inventor, and consultant.